With respect to transport of hydrocarbons through pipelines it is a known problem that the flow of hydrocarbons can be blocked by hydrates and/or ice, the whole cross-section can be blocked by formation of so called hydrate plugs and/or ice plugs. The commonly used method to avoid plugging by hydrates and/or ice in pipelines is to add chemicals to the hydrocarbon flow, which chemicals affect the equilibrium conditions for hydrates and/or ice. The most commonly used chemicals in that respect are methanol and glycols.
To remove hydrate plugs in pipelines it is known to depressurize, as reduced pressure results in a reduction of the hydrate melting temperature, such that hydrates that are formed can melt. However, it is known that the depressurization must be undertaken carefully since ice easily can be formed by said depressurization, such that ice instead of or in addition to the hydrates can block the hydrocarbon flow in the pipeline.
To avoid the above mentioned plugging problems with hydrates and ice it is known to perform direct electric heating of pipelines to maintain the temperature over the equilibrium temperature for hydrate formation and ice formation, respectively, such that hydrates and ice can be avoided. Direct electric heating (DEO) is based on the fundamental principle that electric current in a metallic conductor generates heat due to ohmic loss. Electric cables are connected to the pipeline such that an electric circuit including the pipeline is formed. When formation of hydrates and ice can be expected, the DEO system is set in operation, which takes place by impressing electric alternating current in the electric circuit in which the pipeline is included.
In the Norwegian sector of the North Sea systems have been installed for direct electric heating on six transport pipelines (10″, lengths 6.0-8.5 km) connected to the field Åsgard B and to the condensate pipelines (8″, length 16 km) between the fields Huldra and Veslefrikk. Additionally, for the Kristin field systems have been developed for direct electric heating of six transport pipelines (10″, lengths 6.0-6.7 km). All systems that have been installed have power from the platform infrastructure close to the pipeline systems. The systems for direct electric heating at Åsgard have been commissioned, but no operational experience is so far achieved. However, the systems for direct electric heating on Huldra were set into operation in 2002 and function according to design. The system for direct electric heating on Kristin is planned to be in operation in 2005.
The system for direct electric heating on Huldra has a working voltage of 6.0 kV, and all cables are designed for a voltage U0/U (UM) of 12/20 (24) kV.
However, it has proved to be unduly expensive and comprehensive to implement direct electric heating for pipelines that are situated at large depth or has a smaller degree of thermal isolation, which represents a significant problem.
Therefore a demand exists for a method and a system for direct electric heating for which the above mentioned problem to a large degree is reduced.